[HarshithJeevan-pixel]

Fixes #4883

Description

When entering an empty custom node, returning to the document, and clicking "Preview", the editor panicked with:
panicked at node-graph/graph-craft/src/document.rs: index out of bounds: the len is 0 but the index is 0

This happened because:

1. An empty custom node network has 0 exports, so its original_location.dependants has length 0.
2. replace_node_with_its_exports and flatten_with_fns directly indexed into .dependants[...] without bounds checking.
3. The graph bar and toggle_preview allowed setting preview on a node with 0 outputs.

Changes

• Used .get() and .get_mut() for bounds-safe indexing into OriginalLocation.dependants.
• Guarded toggle_preview and apply_previews against nodes with 0 outputs.
• Hid the "Preview" button in the node graph bar when the selected node has no outputs.
• Added regression tests for flattening nodes without exports and rejecting preview on 0-output nodes.

[cubic-dev-ai]

All reported issues were addressed across 5 files

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[TrueDoctor]

The diff is definetly to big. We should do a targeted fix here and we should not require compiler changes